1. Field of the Invention
This invention relates to ways and means to remove solid materials from liquid wastes.
2. State of the Art
Liquid wastes, such as sewage, from municipal and industrial sources often contain potential pollutants which must be removed before the waste can be disposed of in a receiving water such as a river or lake. It is current practice to remove pollutants from the liquid wastes by treating the waste in an activated sludge-type sewage treatment system.
In a conventional activated sludge-type system the liquid waste is introduced into a primary clarifier which includes a large tank. As the waste travels through the primary clarifier solid particles settle to the bottom of the tank to be removed and disposed of. The clarified waste flows from the tank to an aeration basin, which includes another large tank. Air is mixed with the waste by, for example, conventional surface aerators, as it flows through the aeration basin. The air mixed with the waste dissolves into the liquid, and micro-organisms in the waste utilize the oxygen from the dissolved air in metabolizing organic chemicals in the waste. As the micro-organisms consume oxygen and metabolize organic chemicals they multiply and cluster together so that when the waste leaves the aeration basin it contains particles which include micro-organisms. From the aeration basin the waste flows to a secondary clarifier wherein particles settle to form sludge which is removed, while clarified waste flows to a receiving water. The sludge which settles to the bottom of the secondary clarifier contains substantial quantities of micro-organisms and therefore it is generally termed activated sludge. In a conventional suction-type clarifier a plurality of suction pipes collect the sludge from the bottom of the tank. A part of the activated sludge is returned to the aeration basin, and in this way a high concentration of micro-organisms is maintained in the aeration basin. The remainder of the activated sludge is disposed of.
One type of conventional suction-type secondary clarifier includes a cylindrical tank having a bottom which slopes downward toward the center and a vertical sidewall having an effluent launder disposed around its circumference and near its top. The clarifier also includes a hollow column vertically disposed in the center of the tank for introducing sewage into the tank. Near the top of this center column is a sludge collection box which is connected in communication with a plurality of suction pipes and a draw-off pipe. The suction pipes extend downward from the collection box and radially outward so that their bottom ends are located near the bottom of the tank and spaced apart from one another along the diameter of the tank. The suction pipes and sludge collection box are connected to the center column, and a drive motor and gear system are coupled to rotate the center column, collection box and suction pipes. Thus, in operation the bottom openings of the suction pipes describe circles of different radii concentric to the center column.
The bottom of the sludge collection box is below the level of the surface of the liquid in the tank. Therefore when waste is being clarified in the tank there is a hydraulic head on the sludge on the tank bottom which forces the sludge up the suction pipes and into the collection box. From the collection box the sludge is carried by the draw-off pipe and returned to the aeration basin.
Turning now to the parts of the conventional suction-type clarifier used to introduce waste into the tank, the hollow center column has ports formed in its upper end below the sludge collection box. A cylindrical chamber is concentrically disposed around the upper part of the center column to form a first annular zone around the ports, and the chamber has a plurality of ports formed in its upper part. A cylindrical feedwell is disposed concentrically around the chamber to form a second annular zone which is in flow communication with the tank. In operation, waste flows upward through the center column, out the ports below the sludge collection box and thence into the first annular zone. The waste flows upward around the sludge collection box, through the first annular zone and thence out the ports in the cylindrical chamber and into the second annular zone. The waste then flows downward into the tank.
It should be appreciated that waste often contains grease, oil and other materials, collectively called scum, which float on water. Therefore in conventional practice a plurality of scum ports are formed in the side of the feedwell at the liquid level to permit the scum to flow radially outward from the feedwell to be collected by scum collection means located in the tank outside the feedwell. However, in the suction-type clarifier described above it has been found in practice that the flow pattern of waste introduced below the sludge collection box is not conducive to forcing scum outward through the scum ports, and often scum is trapped within the cylindrical chamber and the feedwell. The trapped scum becomes malodorous and must be removed manually.